Radiation in a Cloudy Atmosphere

Radiative heat transfer is a fundamental factor in the energetics of the terrestrial atmosphere: the system consisting of the atmosphere and the underlying layer is heated by the Sun, and this heating is compensated, on the average, by thermal radia­ tion. Only over a period of 1-3 days from some specified initial moment can the dynamic processes in the atmosphere be considered to be adiabatic. Global dynamic processes of long duration are regulated by the actual influxes of heat, one of the main ones being the radiative influx. Radiation must be taken into account in long-term, weather forecasting and when considering the global circulation of the atmosphere, the theory of climate, etc. Thus it is necessary to know the albedo of the system, the amount of solar radiation transmitted by the atmosphere, the absorptivity of the atmosphere vis-a-vis solar radiation, and also the effective radiation flux, the divergence of which represents the radiative cooling or heating. All these quantities have to be integrated over the wavelength spectrum of the solar or thermal radiation, and they must be ascertained as functions of the determining factors. The relation­ ships between the indicated radiation characteristics, the optical quantities directly determining them, the optically active compo­ nents of the atmosphere, and the meteorological fields will be discussed in this book.

Contenu

I. Cloud Structures and Optical Characteristics of Clouds.- 1. Spatial Structure of Clouds.- 1.1. Cloud frequencies.- 1.2. Structure of stratiform clouds.- 1.2.1. Definitions.- 1.2.2. Height distribution of clouds.- 1.2.3. Horizontal extent.- 1.3. Clouds of vertical extent.- 1.3.1. Definitions.- 1.3.2. Shapes and typical sizes.- 1.3.3. Degree of sky cover and cloud duration.- 1.4. Regional peculiarities of clouds in the Arctic, Antarctic and tropics.- 2. Physical Characteristics of Clouds.- 2.1. Temperature and phase.- 2.1.1. Temperature characteristics of clouds.- 2.1.2. Cloud phase.- 2.2. Cloud water content.- 2.2.1. Definitions.- 2.2.2. Relation of water content to temperature.- 2.2.3. Water content of convective clouds.- 2.2.4. Spatial variability.- 2.3. Droplet-size distribution.- 2.4. Crystals in clouds.- 2.4.1. Shapes of cloud crystals.- 2.4.2. Crystal size and concentration.- 2.5. Microstructural features of polar and tropical clouds.- 3. Space-Time Statistical Structure of a Cumulus Field.- 3.1. A theoretical-experimental model of the statistical structure.- 3.1.1. Mean characteristics of cumulus cloudiness.- 3.1.2. Statistics of cumulus parameters.- 3.1.3. Correlation and spectral characteristics of cumulus fields.- 3.1.4. The use of eigenvectors to describe the cloud cover of alrrrucantars.- 3.2. An empirical model.- 4. Optical Properties of Clouds.- 4.1. Spectrum of optical parameters.- 4.2. An approximate calculation method.- 4.3. Optical parameters of an inhomogeneous cloud for a bimodal droplet-size distribution.- 4.4. Measurements of attenuation of visible light in clouds.- 4.5. Optical characteristics of ice clouds.- 4.5.1. Attenuation coefficient.- 4.5.2. Scattering function.- 4.6. An evaluation of aerosol absorption in clouds.- 4.7. Spectral transmission of thin clouds.- 4.8. Variability of optical thickness for stratiform clouds.- 4.9. Spatial inhomogeneity of optical parameters of clouds.- 5. An Optical Model of the Atmosphere.- 5.1. A model of a cloudless atmosphere.- 5.2. An aerosol model of the atmosphere.- 5.3. Transmission functions of solar radiation in water-vapor absorption bands. Integral transmission function.- 5.4. Transmission function of thermal radiation.- II. Photon Paths in Clouds.- 6. Calculations for Various Cloud Types.- 6.1. Photon-path distribution, mean and effective path lengths in an isolated stratiform cloud.- 6.2. Photon-path distribution and mean lengths in two-layer clouds, taking underlying surface into account.- 6.3. Photon-path distribution and mean path lengths in cumulus clouds.- 6.4. Equivalent trajectories in clouds exhibiting nonuniform absorption.- 7. Experimental Determination of Effective Photon Path Lengths.- 7.1. Measuring effective photon path lengths using a weak absorption band.- 7.2. Results of determining effective photon path lengths.- III. Fluxes of Solar Radiation.- 8. Visible and Ultraviolet Radiation.- 8.1. Methods of calculating albedo, transmission, and absorption.- 8.1.1 Asymptotic methods.- 8.1.2. The Monte-Carlo method.- 8.1.3. Two-stream approximation.- 8.1.4. An improved transport approximations.- 8.2. Albedo and transmittance of homogeneous cloud layers.- 8.3. Radiation properties of ice clouds.- 8.4. Effect of aerosol on radiation characteristics of clouds.- 9. Integral Solar Radiation.- 9.1. Infrared albedo and absorptivity of cloud layers.- 9.2. Integral albedo and absorptivity of cloud layers.- 9.3. Simplified method for calculating fluxes of infrared solar radiation in a cloudy atmosphere.- 9.4. Fluxes and influxes of infrared solar radiation in a cloudy atmosphere.- 10. Experimental Aircraft Studies of Solar Fluxes in the Presence of Stratiform Clouds.- 10.1. Regime of integral solar radiation for St-Sc clouds.- 10.2. Experimental model of "average" St-Sc cloud.- 10.3. Spectral and integral radiation characteristics of stratiform clouds.- 10.4. Radiation properties of urban clouds.- 10.5. Comparison of calculations with data of aircraft measurements.- 11. Solar Fluxes in the Presence of Cumulus Clouds.- 11.1. Radiation regime of isolated cumulus cloud.- 11.2. Statistical structure of fluxes for broken cloudiness.- 11.2.1. Direct radiation.- 11.2.2. Scattered radiation.- 11.2.3. Total radiation.- 11.3. Parametrization of radiation regime of cumulus field using experimental aircraft data.- 11.4. Variability of fluxes of short-wave radiation for broken cloudiness.- 11.5. A comparison of calculated and experimental radiation characteristics of a cumulus field.- 11.5.1. The Monte-Carlo method.- 11.5.2. The analytical method.- IV. Thermal Radiation of a Cloudy Atmosphere.- 12. Methods of Calculating Radiation.- 12.1. Fundamental relations and use of integral transmission function.- 12.2. Thermal radiation of a cloudy atmosphere.- 12.3. Semiempirical methods.- 12.4. Algorithm for models of general circulation.- 12.5. Radiation calculations in cloud-formation models.- 13. Experimental Studies of the Thermal Radiation of a Cloudy Atmosphere.- 13.1. Actinometric radiosonde observations of atmosphere.- 13.2. Actinometric model of the atmosphere.- 13.3. Aircraft studies of thermal radiation of a cloudy atmosphere.- 13.4. Characteristics of an "average" stratiform cloud.- 13.5. Dependence of calculation results on accuracy of measuring atmospheric parameters.- 13.6. Comparison of measurements and calculations under specific conditions.- 14. Effect of Optical Properties of Clouds on Thermal Radiation.- 14.1. Flux distribution inside a cloud layer. Estimates of error of "blackbody" approximation.- 14.2. Thermal albedo of clouds.- 14.3. Emissivity of clouds.- 14.4. Effect of "nonb1ackness" of clouds.- V. Regional Features of the Radiation Regime of a Cloudy Atmosphere.- 15. The Polar Regions.- 15.1. Structural features of the atmosphere.- 15.2. Mean data on radiation regime of cloudless atmosphere.- 15.3. Effect of cloudiness on regime of solar radiation.- 15.4. Effect of cloudiness on regime of thermal radiation.- 16. Radiation Regime of the Tropical Central Atlantic.- 16.1. Fluxes of solar radiation in water-adjacent layer under cloudy and cloudless conditions.- 16.2. Classification of fluxes of thermal radiation.- 16.3. Fluxes of thermal radiation and radiative cooling of a cloudy or cloudless atmosphere.- References.